Neutrino Physics

Why do we exist?

Abandoning the fundamental distinction between matter and antimatter means that the two states can convert to each other. It may also solve one of the biggest mysteries of our universe: where has all the antimatter gone? After the Big Bang, the universe was filled with equal amounts of matter and antimatter, which annihilated as the universe cooled. However, roughly one in every 10 billion particles of matter survived and went on to create stars, galaxies and life on Earth. What created this tiny excess of matter over antimatter so that we can exist?

With Majorana neutrinos it is possible to explain what caused the excess matter. The hot Big Bang produced heavy right-handed neutrinos that eventually decayed into their lighter left-handed counterparts. As the universe cooled, there was insufficient energy to produce further massive neutrinos. Being an antiparticle in its own right, these Majorana neutrinos decayed into left-handed neutrinos or right-handed antineutrinos together with Higgs bosons, which underwent further decays into heavy quarks. Even slight differences in the probabilities of the decays into matter and antimatter would have left the universe with an excess of matter.

It is encouraging that we have seen such a phenomenon recently. In the past three years, the KTeV experiment at Fermilab near Chicago and the NA48 experiment at CERN have established that the neutral kaon — a bound state of a down quark and antistrange quark — and its antiparticle decay in a slightly different manner. At only one part in a million, this difference is very small. However, we only need one part in 10 billion for us to exist. If a similar difference in the decay probabilities exist in right-handed neutrinos, which is quite likely, it could have produced a small excess of primordial matter from which all the other particles have been formed.

1. Introduction
   Neutrinos are everywhere. Trillions of them are passing through your body every second,but they are so shy and we do not see or feel them. They are the least understood elementary particle we know that exist.
2. Birth of Neutrinos
   Existing of neutrinos was suggested as a "desperate remedy" to the apparent paradox that the energy did not appear conserved in the world of atomic nuclei.
3. The Standard Model
   The Standard Model of particle physics can describe everything we know about elementary particles. It says that neutrinos do not have mass. Neutrinos do not have mass because they are all "left-handed" and do not bump on the mysterious "Higgs boson" that fills our entire Universe.
4. Evidence for neutrino mass
   In 1998, a convincing evidence was reported that neutrinos have mass. The Standard Model has fallen after decades of invicibility. The evidence comes from experiments deep underground in pitch darkness with many thousands of tonnes of water housed in mines.
5. Implications of neutrino mass
   Neutrinos are found to have mass, but the mass is extremely tiny, at least million times lighter than the lighest elementary particle: electron. How do we need to change the Standard Model to explain the neutrino mass? Some argue that our spacetime has unseen spatial dimensions, and we are stuck on three-dimensional "sheets". Other argue that we need to abandon the sacred distinction between matter and anti-matter.
6. Why do we exist?
   When Universe started with the "Big Bang", there were almost equal amount of matter and anti-matter. Most of matter was annihilated by anti-matter when Universe cooled. We are leftover of one part in ten billions. Why was there a small excess matter over anti-matter so that we can exist? Once we abandon the sacred distinction between matter and anti-matter, it provides a key to understand why we exist.
7. Outlook The mysteries about neutrinos are now being unraveled dramatically. We will learn much more in the coming years.

This homepage is based on Feature Article "Origin of Neutrino mass" in Physics World, May 2002, by Hitoshi Murayama. The whole article can be download as a PDF file.